[Reference][Forms] move versionadded directives for form options directly below the option's headline

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

When you link directly to a form type option (for example, http://symfony.com/doc/current/reference/forms/types/form.html#action), the ``versionadded`` directive is rendered outside the user's view port which might confuse people. Thus, @WouterJ and me decided to place ``versionadded`` directives for form type options directly below the option's headline (above the ``type`` line).